RARE-EARTH ELEMENT GEOCHEMISTRY OF FERROMANGANESE CRUSTS FROM THE HAWAIIAN ARCHIPELAGO, CENTRAL PACIFIC

The rare-earth element (REE) composition of ferromanganese deposits from the Hawaiian Archipelago was determined as part of a geochemical investigation into the origin and environment of deposition of crusts. Most samples exhibit shale-normalized REE patterns which are typical of hydrogenous ferromanganese deposits. Patterns of two todorokite- and manjiroite-rich crusts are significantly different from those of the other samples. The range of concentrations in typical Hawaiian crusts. (sum of REE 0.15-0.25 wt.%) is in the uppermost portion of that reported for deep-sea Fe-Mn nodules. Shallow-water (< 2000 m) crusts exhibit lower REE concentrations than those from deep water (> 2000 m) as well as slightly greater REE fractionation. Large positive Ce anomalies attributed to the oxidative scavenging at the MnO2 surface are observed on all samples except one. The slight light REE (LREE) depletion in shallow-water crusts relative to deeper samples is discussed in terms of variations in the REE concentrations in the water column and the relative complexing behavior of the REE. No significant trend in Ce concentration with depth is observed. Partition coefficient calculations suggest that concentrations of trivalent REE in crusts are affected by their availability in seawater to a greater extent than by the composition of the scavenging ferromanganese oxide flocs.